This invention relates to variable capacity roller- and vane-type pumps. The rollers and vanes in such pumps operate as piston elements.
According to this invention there is provided a variable capacity pump incorporating inlet and outlet ports and comprising a rotary carrier having slots at its periphery, piston elements mounted in the slots for radial movement, a cam ring encircling the carrier the radially inner surface of which is engaged by the piston elements to pump working fluid from the inlet port to the outlet port of the pump, a casing within which the cam ring is mounted for guided movement to adjust the position of the cam ring relative to the axis of rotation of the carrier and hence the output of the pump, resilient means urging the cam ring into a position in which the quantity of fluid delivered is a maximum and means defining between the casing and the cam ring a chamber communicating with said outlet port, the fluid pressure in said chamber acting on the cam ring in opposition to the spring, and means whereby a damping force is applied to movement of the cam ring which damping force varies in dependence upon the instantaneous position of the cam ring.
Preferably, the damping force increases with movement of the cam ring to increase the output of the pump.
In one arrangement according to the invention a passage communicating with said outlet port opens to said chamber through a venting port which is obstructed to a variable extent by the cam ring in its guided movement, thereby to provide said means for applying a variable damping force.
In alternative arrangements the means for applying the damping force is independent of the supply of pressure fluid from said outlet port. In one such arrangement said means comprises a tapered recess opening to the chamber and a tapered piston connected to the cam ring and disposed in the recess so that said guided movement of the cam ring causes the radial clearance between the piston and the wall of the recess to vary and impose a variable restriction on the flow of the fluid into and out of the recess .